Mooning

Dave North

This month we have a real show going on, right in prime time! At about five minutes after 10 pm on October 18, the moon will occult Uranus from all of our Northern California sites (youse folks that are further away might want to run your own numbers on this).

It will disappear near the north pole on the dark side, and reappear about 45 minutes later a bit further south on the bright side. Both of these events should be great fun to watch.

Uranus is a bit brighter than Mag 6, so it shouldn't be hard to watch on the disappearance, but you may find the reappearance a bit more challenging.

The great thing about this is you couldn't ask for a better time schedule; you won't even have to lose much sleep to catch the whole thing.

And as a bonus, it's a great night for mooning anyway, with the terminator nearing Copernicus ... all the great Appenine features showing. Mmm!

One thing to look for: when near the moon (an almost monochrome body) the color of Uranus (no jokes now!) should be very apparent. Make sure you note what you see and compare.

That's a tough act to follow, but there are a couple of other things to note in October.

For early risers, the predawn moon at the beginning of the month will be spectacular. It will be very, very high in the sky (almost overhead!) for great seeing, and nearing first quarter.

If you do get up early, this is your time of year for some great views.

And, for those who missed the other opportunities to scope out Mare Orientale, there's another, lesser shot on the 23rd. Probably won't be a really good view due to a weak libration, but worth a glance (what else is there to do on a full moon Saturday?)

...which brings up this month's extended topic:

Libration.

What is this "libration" I'm always talking about?

First, it isn't "liberation" (end of the school year) or "libation" (though enough "libations" and "librations" seem to start happening all over the place).

The word "libration" is rooted in the word for "scale" (as in libra). What it refers to is the oscillation or rocking motion in the apparent aspect of a secondary body (as a planet or a satellite) as seen from the primary object around which it revolves.

This is, as far as I know, its only meaning.

So it's easy to see why you normally only hear this word used in reference to the moon: from our point of view, there's only one satellite we're likely to be looking at in enough detail to see this "oscillation."

What does it mean to the observer? It means the moon doesn't present exactly the same face to us all the time, but rather it rocks back and forth - and up and down. Each motion has its own "reason."

Sometimes, then, we can see more of one side or the other, and when that side is well lit, we're in business: we can see things that are normally not visible. The same is true in the north and south, of course.

Because of the way the terminator moves, all of the directions are best seen near the full moon. Near full, more of the poles are lit and a longer stretch of polar terminator is visible.

Just before full, the terminator will be near the western side. Just after, near the east.

The notable exception to these rules is when you're looking for contrast between maria (seas; the smooth dark areas) and highlands. This is best seen in high light, so will be true for the eastern limb at first quarter, and for the western limb at third quarter. So a strong east or west libration near those times is always worth a look, too.

Why does the moon rock and roll?

Mostly, it doesn't. It does wiggle a little bit, but that hardly shows.

We see the top and bottom more because the orbit of the moon is inclined to our equator (in fact, it is fairly close to the ecliptic, varying about +-5 degrees).

When it is higher than our equator, we get a bit of a glance at the bottom. When lower, we see more of the top. As you can see, the view varies slightly for people in, say, Australia than from here.

What about east and west?

This is a bit more interesting to me. The moon has an ellipitcal orbit (as do all natural bodies; a perfect circle would be extremely unlikely).

In an elliptical orbit, any body goes faster at times (when closest to the body it orbits) and slower at others. But the Moon rotates at a fixed, regular rate - even if it's only once per orbit.

That means its rate of turn per mile travelled is smaller when it's near the earth, and higher when it is further away. Presto! The moon rocks.

(Try this with a tennis ball or something; just move it around anything while duplicating those two effects, and you'll see the libration).

Of course, it's a little more complicated than that, but those are the main ideas involved... and now you can tell all your coworkers about lunar librations, and if you say the whole thing fast enough, watch their eyes spin...

Oh, one last note: the moon almost never actually "faces" the earth at all! Its face points toward the other focus of the ellipse (where the earth isn't) and drifts back and forth around that focus. By force, this sweeps across the earth twice each orbit (near apogee and perigee) but since it's usually looking either north or south of us (just slightly) we usually aren't in its crosshairs.

How often are we? Turns out to be about the same frequency as eclipses (since the two effects are similar in nature).